Internal combustion engines are supplied with a mixture of air and fuel for combustion within the engine that generates mechanical power. To maximize the power generated by this combustion process, the engine is often equipped with a turbocharged air induction system.
A turbocharged air induction system includes a turbocharger having a turbine that uses exhaust from the engine to compress air flowing into the engine, thereby forcing more air into a combustion chamber of the engine than a naturally aspirated engine could otherwise draw into the combustion chamber. This increased supply of air allows for increased fueling, resulting in an increased engine power output.
The fuel energy conversion efficiency of an engine depends on many factors, including the efficiency of the engine's turbocharger. Previously proposed turbocharger designs include shafts connecting the turbine and compressor wheels that are as short and as stiff as practical to avoid vibrations and other dynamic effects during operation. It should be appreciated that a turbocharger shaft may rotate at tens of thousands of revolutions per minute during normal operation, thus requiring a well balanced and stiff assembly.
However, for certain applications, for example, mixed flow turbines in which gases drive the turbine wheel both axially and radially, the entire rotating assembly that includes at least a shaft and a turbine wheel must grow longer. To maintain stiffness, the longer shaft is typically also made thicker, which increases the mass of the rotating assembly and thus decreases its transient response with respect to a shorter, and thinner, turbine shaft, which is generally not desirable.